Silicon carbonitride with spinel structure

ABSTRACT

A compound having a space spinel structure and the formula Si3-x Cx N4 wherein 0&lt;x≦1. An example of the compound is spin el silicon carbonitride. The compound of the invention may be made by providing a silicon carbo-diimide compound and subjecting the compound to elevated temperature and pressure conditions.

BACKGROUND OF THE INVENTION

This invention relates to a hard material.

Ultra-hard materials may be defined as those materials which have aVickers pyramid hardness greater than 40 GPa. Examples of such materialswhich currently find extensive use in industry are diamond and cubicboron nitride. These two ultra-hard materials are not suitable for allapplications, despite their hardness. For examples, diamond is a poorabrasive for many iron materials as iron tends to react chemically withthe diamond under abrasive conditions. There is thus a need for anultra-hard material with a chemical composition different from that ofdiamond and cubic boron nitride. The literature contains references totheoretical predictions and suggestions as to such materials.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided acompound having a spinel structure and the formula:Si_(3-x)C_(x)N₄ wherein 0<x≦1  (1)

A spinel structure, as is known in the art, has the space group Fd3m(International Tables for X-ray Crystallography 1952, pages 340 and341).

According to another aspect of the invention, there is provided acompound of the formula (1) for use in an abrasive, tribological,thermal, optical, dielectric, electronic and/or optoelectronicapplication.

According to yet another aspect of the invention, there is provided amethod of making a compound of formula (1) which includes the steps ofproviding a silicon carbo-diimide compound and subjecting the compoundto elevated temperature and pressure conditions to convert the siliconcarbo-diimide into a compound of the formula (1).

DESCRIPTION OF EMBODIMENTS

The invention provides a compound of formula (1) having a spinelstructure and which is an ultra-hard crystalline material having ahardness greater than 40 GPa. Thus, the material has particular use asan abrasive in applications such as grinding, cutting, milling, boringand as a wear surface. Furthermore, the material has a high conductivityand has a low dielectric constant and thus has use as a dielectricpassivation layer in electronic devices. The material has semi-conductorproperties and has use in active electronic and optoelectronic devices.

The compounds of formula (1) may be doped with other elements. Thus,some of the silicon, which is located at 16d* site within the spinelstructure, can be substituted by heavier atoms of groups IVa and IVb ofthe Periodic Table, e.g. germanium, tin, lead, titanium, zirconium andhafnium. Typically, such an atom or element, when it occupies a siliconsite, will be present in an amount up to 12 percent by number of siliconsites. The dopant may also be an element such as oxygen which willoccupy an anion site located at 32e*. When the element occupies an anionsite, then the element will typically be present in an amount of up to 9percent by number of anion sites. A resulting charge deficiency will becounteracted by cation vacancies. [* Wyckhoff notation, seeInternational Tables of Crystallography (1952), p. 340 and 341].

The strongly covalent nature of the C—N bonding in the compound of theinvention enhances the hardness of the material. Further, it is believedthat the compound is protected from oxidation by a passive SiO₂ layer onits surface.

The compounds of formula (1) may be produced by a method which involvesconverting a silicon carbo-diimide compound under elevated temperatureand pressure conditions for the desired compound. The elevated pressurewill typically be greater than 10 GPa and the elevated temperature willtypically be greater than 1000° C.

The silicon carbo-diimide starting material may be a silicon(carbo-diimide) nitride, described by the formulaSi₆N_(8-2x)[N═C═N]_(3x) wherein 0<x<1, or the crystalline phase,Si₂N₂[N═C═N]. The last-mentioned compound is the preferred startingmaterial. These starting materials may be produced by a method describedin Reference 1 or by a variation of such method.

When the compound produced is doped with a metal in a cation site it ispreferred that the element be introduced into the starting materialbefore the conversion step. Similarly, if the compound produced is onewith an element such as oxygen in an anion site, the element is againpreferably introduced into the starting material before the conversionstep.

The invention will now be illustrated by the following examples.

EXAMPLES Example 1 Spinel Silicon Carbonitride (Si₂CN₄) from Silicon(Carbon-diimide) Nitride

A multi-anvil high-pressure apparatus equivalent to the one described,for example, by Ruble (Reference 2) and Walter (Reference 3) was used inthis example.

Silicon (carbo-diimide) nitride, produced according to the methoddescribed in Reference 1, was ground to a powder in inert gas, placedinto a precious metal pipe (Re or PT, diameter: 1.5-1.7 mm, height: 2.7mm) and compressed along one axis with a plunger. The precious metalpipe was sealed airtight and placed in an octahedral pressure transfercontainer made of MgO (length of edges 10 mm). Concentrically, aroundthe sample was an electrical insulation layer of hexagonal BN, a heatingunit made of LaCrO₃ and a ZrO₂ pipe. The temperature was measured bymeans of a W₉₇ Re₃-W₇₅ Re₂₅ thermocouple that had been coaxiallyinserted into the heating unit. An MgO octahedron containing the samplewas placed between 8 hard-metal cubes (25 mm length of edges 8 mm) andcompressed within a hydraulic press. The pressure inside the sampleconfiguration was correlated with the hydraulic pressure of the pressvia phase transitions. The pressure applied to the sample was 18±1 GPa.

The sample, under the pressure indicated above, was heated for 15minutes to a temperature of 1800° C., held there for 20 minutes andcooled to 100° C. in 1.5 minutes by switching off the heating. Afterdecompression, the sample material appeared as a solid, colourless,opaque body. Analysis with micro Taman spectroscopy and X-ray powderdiffractometry did not detect any phases apart from the precious metalused for encapsulation and c-Si₂CN₄, i.e. spinel Si₂CN₄.

Example 2 Spinel Silicon Carbonitride from Silicon (Carbo-diimide)Nitride with Oxygen Doping

Silicon tetrachloride and bis-(trimethyl-silyl)-carbo-diimide withcatalytic amounts of pyridine were converted to the correspondingsilicon carbo-diimide starting material according to the methoddescribed in Reference 1. The generated trimethyl-chlorosilane wasremoved via distillation. The raw material thus produced was filledunder inert gas into a combustion boat made of quartz glass and calcinedin a quartz pipe in vacuum at 350° C. in vacuum to remove the remaininggases and solvent traces. The quartz pipe was subsequently flooded withair instead of argon. After 30 minutes (at an air humidity of 17%) thematerial was heated in an argon stream of 2 cm³/min and 100 K/h to 960°C. After 5 minutes the material was left to cool to room temperature.The cyanamide that was generated through hydrolysis by the humidity ofthe air was removed during the heating phase and condensed in the coolerpart of the quartz pipe. The silicon (carbo-diimide) nitride producedthis way had a relatively high oxygen content (approximately 4.5% massaccording to ultimate analysis). The white product was ground andhomogenised with an agate mortar in inert gas and introduced into theMgO pressure transfer container as described in Example 1. Theapproximate elemental composition in the produced spinel siliconcarbonitride was determined with electron probe micro-analysis (EPMA).The values are given in percent by mass: Si 46%, C 8%, N 41%, O 5%.

Example 3 Spinel Silicon Carbonitride from Silicon (Carbo-diimide)Nitride Doped with 1% Atom Titanium

A mixture of silicon and titanium tetrachloride in a mass ratio of 98.3:1.7 with bis-(trimethyl-silyl)-carbo-diimide and catalytic amounts ofpyridine was converted to the corresponding silicon titaniumcarbo-diimide according to the method described in reference 1. Viaappropriate temperature treatment, which is also described in Reference3, this material was converted to silicon titanium (carbo-diimide)nitride with the composition Si_(1.98)Ti_(0.002)CN₄. The conversion ofthis material to the Ti-doped c-Si₂CN₄ was carried out as described inExample 1. A lower pressure of 15 GPa was applied.

Example 4 Spinel Silicon Carbonitride with Increased Silicon Content

Silicon tetrachloride with a mixture ofbis-(trimethyl-silyl)-carbo-diimide and bis-(trimethyl-silylamine in amass ratio of 85:15 with catalytic amounts of pyridine was converted tothe corresponding silicon (carbo-diimide) nitride according to themethod described in Reference 1. The generated trimethyl-chlorosilanewas removed via distillation. This is described by the idealisedequation below:2.4 SiCl₄+2.8 ((CH₃)₃Si)₃N+0.6 (CH₃)₃Si—N—C—N—Si(CH₃)₃↓pyridine (catalyst)Si_(2,4)C_(0,6)N₄↓9,6 (CH₃)₃SiCl↑

The silicon (carbo-diimide) nitride was calcined as in Example 2, butwithout allowing exposure to air. The conversion of the resultingproduct to a compound with the approximate composition Si_(2.4)C_(0.6)N₄having the spinel structure was carried out as described in Example 1. Alower pressure of 15 GPa was applied.

Example 5 Spinel Silicon Carbonitride Using Shockwave

The process set out in Example 1 was repeated, save that the elevatedtemperature and pressure conditions applied were a temperature of 1200°C. and a pressure of 20 GPa. These conditions were maintained for aperiod of 15 minutes. A hard product was produced and was believed to bespinel Si₂CN₄.

References

-   1. R. Riedel, A. Greiner, G. Miehe, W. Dressier, H. Fuess, J.    Bill, F. Aldinger; Angew. Chem. Int. Ed. Engl. (1997), 36, 603-606.-   2. D. C. Ruble; Phase Transitions (1999), 68, 431-451.-   3. M. J. Walter, Y. Thibault, K. Wei, R. W. Luth; Can. J. Phys.    (1995), 73, 273-286.

1. A compound having a spine structure and the formula:Si_(3-x)C_(x)N₄ wherein 0<x≦1.  (1)
 2. A compound according to claim 1which is doped with another elernent.
 3. A compound according to claim 2wherein the dopant element substitutes some of the silicon.
 4. Acompound according to claim 3 wherein the dopant element is an atom ofgroup IVa and IVb of the Periodic Table.
 5. A compound according toclaim 4 wherein the atom is selected from germanium, tin, lead,titanium, zirconium and hafnium.
 6. A compound according to any one ofclaims 3 to 5 wherein the dopant occupies a silicon site and is presentin an amount of up to 12% by number of silicon sites.
 7. A compoundaccording to any one of claims 2 to 6 wherein the dopant element isoxygen and occupies an anion site.
 8. A compound according to claim 7wherein the oxygen is present in an amount of up to 9% by number ofanion sites.
 9. A compound according to any one of the preceding claimsfor use in an abrasive, tribological, thermal, optical, dielectric,electronic or optoelectronic application.
 10. A method of making acompound of any one of claims 1 to 8 which includes the steps ofproviding a silicon carbo-diimide compound and subjecting the compoundto elevated temperature and pressure conditions to convert the siliconcarbo-diimide into a compound of the formula (1).
 11. A method accordingto claim 10 wherein the elevated pressure is greater than 10 GPa and theelevated temperature is greater than 1000° C.
 12. A method according toclaim 10 or claim 11 wherein the silicon carbo-diimide is a silicon(carbo-diimide) nitride having the formulaSi₆N_(8-2x)[N═C═N]_(3x) wherein 0<x<1.
 13. A method according to claim10 or claim 11 wherein the silicon carbo-diimide is a silicon(carbo-diimide) nitride having the formulaSi₂N₂[N═C═N].
 14. A compound of claim 1 substantially as hereindescribed with reference to any one of the examples.
 15. A method ofclaim 10 substantially as herein described with reference to any one ofthe examples.